Rotatable windows that are automatically cleaned by a stationary cleaning element (e.g., wiper) without obscuring an observer's field-of-view are known per se. Reference is made, for example, to JP56169475, Dec. 26, 1981, which discloses a device for cleaning a TV camera comprising a rotating plate, a lower part of which is immersed in a water tank and which is rotated by an output shaft of a geared motor. A water-proof seal is to provided between the plate and a camera housing to avoid the water immersion in the camera housing. Current problems that militate against the implementation of rotatable windows and stationary cleaning elements for use with electro-optical assemblies include the high friction of the window's dynamic sealing element, commonly used to protect the instrument housing from environment influences, and the high power and complexity of the driving line. In conventional configurations, where the dynamic sealing element is located between the window and the housing, the larger the diameter of the window, the higher is the friction and therefore the higher the power and the complexity of the driving line. Owing to the necessity of keeping the optical window and the housing of the cleaning device sealed, the possibility of reducing the friction in conventional configurations is limited.
Another problem associated with the cleanliness of moisture-sensitive equipment in general, and optically transparent devices in particular, relates to the need to protect the equipment from the effects of moisture. One way to accomplish this is to seal the equipment housing. However, the sealed housing may be exposed to significant pressure differentials driven by temperature and/or altitude variations thus requiring the housing/container to be built from suitably strong material and adding to its cost. To avoid heavy and costly packaging, breather valves are commonly used. These valves prevent excessive pressure differentials caused by temperature and/or altitude variations, but allow ambient air to enter into the interior of the housing/container.